Ferro-electric devices are employed as part of an electronic (e.g. computer) memory. Their function is based on an electronically programmed binary state (a “0” or a “1”), which can be read out electronically as well. A plurality of such devices is used to form a memory. The programming requires a voltage to polarize the ferro-electric material.
Non-volatile memory devices have the desired property that they will not loose their programmed state off-power. Conventional, non-volatile memory devices are not, however, intrinsically non-volatile. In the context of the invention, this means that the voltage needed for read-out will not change the polarized state of the device. I.e., each read-out of a device which is not intrinsically non-volatile, will have to be followed by step in which the device is polarized back again into the original state. This is a drawback, particularly in view of the limits the repeated poling puts on the life of the device.
A reference in this regard is a paper by Blom et al., Physical Review Letters, Volume 73, Number 15, 10 Oct. 1994, 2109, which describes a Schottky diode. More precisely, Blom describes a two-terminal memory device comprising
a first electrode;
a second electrode; and, adjacent to said electrodes,
a film of a ferro-electric semiconductor material, viz. PbTiO3.
In this device, the binary states are determined by a high or a low conductance state of the ferro-electric semiconductor. This is neither practical, nor workable, as the conducting properties go at cost of the ferro-electric properties, i.e. this leads to a device which either cannot be polarized or cannot be read-out.
It is an object of the invention to provide an electrical switch which is intrinsically non-volatile. The high and low conductance state serve as the binary ‘1’ or ‘0’ (or vice versa) in a memory application, or as an active electrical switch to select, e.g. a display pixel. Further objectives are to increase storage density, to decrease the processing temperatures such that the fabrication of the non-volatile electrical switch is compatible with back-end silicon, to reduce the processing costs, and to realize an a-symmetrically switching diode (that is always in reverse in one bias direction).
Moreover, it is an object of the invention to provide a modulatable injection barrier (in effect mostly a modulatable electrode) that can be used not only in memory devices and switches, but also in other devices such as three terminal (field-effect) devices, and rectifying or light-emitting semiconductor diodes. More particularly, it is an object of the invention to provide organic semiconductor diodes which allow more than one programmed state (i.e. creating an asymmetrically switching diode).
Another object of the invention is to provide memory devices, notably if based on organic materials, which are capable of avoiding crosstalk.
As a further background on the use of organic semiconductor materials, the following references are referred to. Cheng Huang et al., in Electrets 2005, pages 91-94, provides a nanocomposite having a high dielectric constant. Herein a high dielectric matrix polymer is provided with polyaniline conductive filler. With reference to the conductive filler concentration being close to the percolation concentration, the aim is to obtain a material of high dielectric constant that is particularly useful for its electromechanical response. The reference therewith does not relate to a modulatable injection barrier, as the aim of the document goes against obtaining a state of conductivity. Similar considerations hold for a reference by the same first author, in Applied Physics Letters, 87, 182901 (2005), pages 182901-1-1982901-3.
Background references on memory elements using ferro-electric polymers 35 are WO 2006/045764 and WO 02/43071.